Numerous processes are employed in the field of exhaust gas purification, with wet processes being generally distinguished from dry processes.
Typically, dry purification processes include the blowing of powdery calcium hydroxide into the exhaust gas stream in order to neutralize noxious acidic substances, such as sulfur dioxide, hydrogen chloride and hydrogen fluoride, so that the thus formed neutral salts can be conveniently removed by means of appropriate separating devices, such as electrostatic filters and fabric filters. Dry purification also typically includes semi-dry purification, wherein a suspension of calcium hydroxide in water, is sprayed into hot exhaust gas such that the wet calcium hydroxide is dried and reacts with substances within the gas.
The dry purification of exhaust gas is performed in numerous various industries. Primary fields of application are the purification of exhaust gases from coal and lignite power plants, refuse incinerating plants, incinerating plants for special refuse and a variety of other facilities wherein the burning of fuel materials creates pollutants.
By applying a dry exhaust gas purification process, a high percentage of noxious substances, such as for example HCL, HF and SO.sub.2, can be removed from the exhaust gas. One continuing problem however, is that the process consumes significant quantities of calcium hydroxide for the amount of pollutants removed.
In general, the stoichiometric factor for calcium hydroxide use ranges from 3.5 to 6 times the stoichiometric quantities theoretically required for the amount of pollutant to be removed. In refuse incinerating plants, for example, instead of the 10 to 12 kg of calcium hydroxide estimated to be used per kilogram of exhaust gas pollutants, an amount of more than 30 kg of calcium hydroxide is generally consumed. This low efficacy of the dry exhaust gas purification process appears to be due to the fact that the individual calcium hydroxide particles do not react completely through their entire mass. A layer of reaction products appears to form on the surface of the calcium hydroxide particle, and as a result, the noxious acidic substances to be removed are prevented from penetrating further into the calcium hydroxide particle. Thus it is desirable to increase the reactivity of calcium hydroxide for use in exhaust gas treatment processes.
By an increase in the reactivity of Ca(OH).sub.2 it is meant that there is a reduction in the amount of Ca(OH).sub.2 required to achieve a specific degree of removal of the noxious acidic substances. A low stoichiometric factor of Ca(OH).sub.2 to noxious acidic substance is indicative of an increased reactivity.
Various attempts have been made in the past to reduce the calcium hydroxide consumption and increase the particulate efficiency. One means to achieve increased efficiency is to reactivate the exhausted calcium hydroxide particles, after they have been used in the purification of an exhaust gas, by mechanically reprocessing it through subjecting the particles to a grinding treatment sufficient to separate the outer unreactive layers from the inner, potentially still reactable, mass to stimulate efficiency. In another attempt to achieve increased efficiency, the reaction product is stored for one or two days, after which period it is then used again. Apparently, the stored particles reinvigorate their efficacy, to some extent, and have somewhat regenerated activity.
Generally, the efficacy of each of the above-mentioned processes, with respect to an increase in the reactivity of calcium hydroxide, has been found to be too low to be commercially satisfactory.
Thus, there has been a continuing commercial need, to provide calcium hydroxide compounds having an increased reactivity to the noxious acidic substances contained in the exhaust gases. Further, it is also extremely desirable to remove additional substances from the exhaust gas that might not be satisfactorily removed by existing calcium hydroxide compounds, particularly nitrogen oxides, volatile heavy metals such as mercury, cadmium, arsenic and the like, and preferably removal of organic substances such as chlorinated dioxines/furanes, chlorinated hydrocarbons, polychlorinated biphenyls (PCB's) and polycondensed aromatic hydrocarbons (PAH), in a single step together with the other noxious acidic substances and without the requirement of separate process steps.
Still further, it is commercially desirable for the carbon monoxide content and/or the total carbon content of the treated exhaust gas to be reduced, as the noxious acidic substances are being removed.
These and other needs and desires are addressed by the invention described as follows.